A wearer may be prescribed a positive or negative optical power correction. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition representing the power increment between far vision and near vision. The addition is qualified as prescribed addition. Ophthalmic lenses suitable for presbyopic wearers are multifocal lenses, the most suitable being progressive multifocal lenses.
Actually, the vision perception of a lens wearer is not limited to the objects which are located along the gaze direction of the wearer at one moment. The wearer also perceives simultaneously objects which are shifted transversally with respect to the gaze direction, although these latter objects are not fixated by the wearer. The whole range of the visual perception of the wearer relating to one and same gaze direction has therefore a non-zero transverse extension, which is called perceptual span in the art.
But the perceptual span is not distributed symmetrically with respect to the gaze direction. This is so in particular when the wearer is reading. Indeed, it is well-known that the perceptual span extends further parallel to the text line in the direction of the text to be read just after that located along the gaze direction. Put another way, the whole vision field extends further in the direction of the next line portion to be read, from the current gaze direction, as compared to the direction of the line portion already just read.
However, current lens designs do not take into account such perceptual span.